The dominant absorption process for gamma rays of about 0.05 to 30 MeV is the Compton interaction. At present the best method to detect these gamma rays is the Compton double scatter technique since single Compton scattering alone gives neither the direction nor the energy of the incident gamma ray. In contrast, the Compton double scatter technique yields both the direction and the energy of the incoming photon.
Present detectors which use the Compton double scatter technique determine the direction of the incoming photon to a ring in the sky since the direction of the recoil electron at the first scatter cannot be measured. Time-of-flight measurements are normally used to discriminate between gamma rays coming through the field-of-view and those entering through the back of the system. Typically this requires that the first scatterer (ie., the hodoscope) is separated from the second scatterer (i.e., calorimeter) by approximately 1.5 meters.
Emission computed tomography (ECT) and associated technologies are mainly used for the detection and imaging of the radiation produced by radiotracers and radiopharmaceuticals. The primary application for ECT systems is in medical study and diagnosis due to the potential for imaging organ functions in real time. The two major ECT instruments presently used are Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET).
Positron scanners for use in locating brain tumors were first developed in the early 1960s with the first PET system completed in 1975. Pet systems have become an essential medical diagnostic tool for a variety of reasons. For one, very high efficiencies utilizing positron emitting labels can be achieved through the coincidence collimation of the annihilation radiation. Another advantage is that the common radiopharmaceuticals used with PET systems typically have very short life times, thus allowing large doses to be administered to a patient as well as the performance of repetitive studies. Recently, the utilization of photon time-of-flight information with fast scintillators has improved the SNR that can be obtained in images of the distribution of positron emitting radionuclides.
Present PET systems use bismuth germanate oxide (BGO) crystals. BGO crystals have the highest effective atomic number and stopping power of any scintillator crystal available today. This translates into a higher photopeak fraction and a lower Compton continuum than other crystals such as NaI and BaF.sub.2. Gadolinium orthosilicate (GSO) crystal is an alternative which has a slower decay time but larger pulse yield than BGO. PbCO.sub.3 crystals nearly equal the stopping power of BGO but the light output is about 10 times lower.
From the foregoing, it is apparent that an improved imaging system for use with gamma rays, x-rays, and positrons is desired.